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One-color two-photon ionization of methyl iodide, resulting in the CH3I+ and CD3I+ ions in the 2E3/2 and 2El/z electronic states, is investigated using the zero kinetic energy (ZEKE) photoelectron technique. Unlike the conventional photoelectron spectra, the two-photon spectra are resonantly enhanced via the dissociative A-state continuum and exhibit an extensive vibrational structure. A long progression in the C-I stretching vibration ug+ is observed, and reliable values for all the vibrational frequencies of the ions and for the ionization potentials are obtained. The appearance of the spectra is discussed in terms of consecutive two-photon absorption via a dissociative intermediate state and a time-dependent picture of spectroscopy. Experimental evidence for the influence of the asymmetric V6 vibration on the photodissociation process is presented. It is shown that two-photon ZEKE spectroscopy via a dissociative intermediate state can serve as a powerful probe of the photodissociation dynamics. It also allows one to explore regions of the ground-state potential energy surface of the ion that are far from equilibrium.

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The hydrogen-bonded phenokthanol complex has been studied using both two-color resonant-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) photoelectron spectroscopy. The REMPI spectrum shows vibrational structure which can be assigned to low-frequency intermolecular modes on the SI origin and in combination with intramolecular (phenol-localized) vibrations of the S1 electronic state: a reinterpretation of the low-energy region of the REMPI spectrum gives the frequency of the intermolecular stretch in the S1 state as 162 cm-I. ZEKE spectra were recorded using different vibrational excitations of the SI state as the intermediate level. These spectra show different vibrational excitation of the ion due to modified Franck-Condon factors for the ionization step. The ZEKE spectra allow all six intermolecular modes of the phenokthanol cation to be determined. The ionization energy is determined accurately as 62 901 & 5 cm-1 [7.7988 f 0.0006 eV] and the increase in binding energy between the SO and the ionic ground state is derived as 5727 f 10 cm-l, giving an approximate value for the ionic complex bond energy of 8000 cm-I [ca. 1 eV].